The act of pressing one's thumb (or finger) into the rectum of a woman while engaged in vaginal intercourse from the doggystyle or reverse cowgirl position.
Hint: A tom thumb should never be attempted on a male without his expressed permission.

"My date didn't go so well last night..."
"Shoulda given 'er the tom thumb dude. You know--stick in a thumb, pull out a plum? Chicks LOVE that shit!"

verb. The action of placing one's thumb as far up another individuals rectum as possible. Often results in extreme surprise and discomfort for the individual on the receiving end of the thumb, and great pleasure for the attacker. In some variations, once the thumb is firmly placed inside the anal cavity, the attacker will go even further by quickly moving his thumb around in a circular motion inside the poop shoot(Tom Thumb Twister). Many prefer to lick their thumbs on the pre-insertion approach to make the execution of the Tom Thumb go more smoothly. However, it's not advisable to lick one's thumb post-insertion.